1. Field of the Invention
The present invention relates to a crystalline thin-film semiconductor and a manufacturing method thereof. The invention also relates to a semiconductor device using the above thin-film semiconductor and a manufacturing method thereof.
2. Description of Related Art
Techniques are known in which a crystalline silicon film is formed on a glass or quartz substrate and thin-film transistors (hereinafter referred to as TFTs) are formed by using the silicon film. Such TFTs are called high-temperature polysilicon TFTs or low-temperature polysilicon TFTs.
In the case of high-temperature polysilicon TFTs, a crystalline silicon film is formed by a technique including a heat treatment at a relatively high temperature of 800–900° C. It can be said that this technique is derived from an IC manufacturing process using a single crystal silicon wafer. Naturally, high-temperature polysilicon TFTs are formed on a quartz substrate, which withstand the above-mentioned high temperature.
On the other hand, low-temperature polysilicon TFTs are formed on a glass substrate, which is inexpensive but is apparently lower in heat resistance than a quartz substrate. To form a crystalline silicon film for low-temperature polysilicon TFTs, heating at lower than 600° C. which a glass substrate can withstand or laser annealing which causes almost no thermal damage on a glass substrate is performed.
The high-temperature polysilicon TFT is advantageous in that TFTs having uniform characteristics can be integrated on a substrate.
On the other hand, the low-temperature polysilicon TFT is advantageous in that a glass substrate can be used which is inexpensive and can easily be increased in size.
According to the current manufacturing techniques, there are no large differences in characteristics between the high-temperature polysilicon TFT and the low-temperature polysilicon TFT. That is, in both cases, the mobility is 50–100 cm2/Vs and the S value is 200–400 mV/dec. (VD=1 V).
However, these values are much worse than those of MOS transistors formed on a single crystal silicon wafer. In general, the S value of MOS transistors formed on a single crystal silicon wafer is 60–70 mV/dec.
At present, there are active matrix liquid crystal display devices in which an active matrix circuit and peripheral driver circuits are integrated on the same substrate by using TFTs. In this type of configuration, the source driver circuit of the peripheral driver circuits is required to operate at a frequency higher than a little more than 10 MHz. However, at present, a circuit using high-temperature polysilicon TFTs or low-temperature polysilicon TFTs can provide a margin of operation speed that is as small as several megahertz.
For this reason, at present, a liquid crystal display device is constituted by dividing its operation (called “divisional driving”). However, this method has several problems; for example, stripes appear on the screen due to, for instance, a slight deviation in the division timing.